[Parathyroid hormone related-protein and calcium homeostasis]

Recently, parathyroid hormone-related protein (PTHrP) was identified as one of the major causes of humoral hypercalcemia of malignancy in several species. The hormone probably has an important role in the physiology and pathophysiology of mammals and birds. Many endocrine, paracrine and autocrine functions are attributed to PTHrP. Parathormone shares the same receptor with PTHrP. This receptor was isolated in many tissues. PTHrP could be an important fetal growth factor. The influence of PTHrP on calcium homeostasis is presently the object of active research. PTHrP stimulates calcium transfer through the placenta and maintains a concentration gradient between the dam's blood and the fetus. The hormone is produced in large quantities in milk. However, its exact and principal function in lactation has not yet been determined. Among other effects, PTHrP might stimulate the secretion of calcium, phosphate and magnesium in milk and might foster the development of the mammary gland. A role of PTHrP in the pathogenesis of postparturient paresis in dairy cows has been hypothesized. Results of recent trials demonstrate that despite an important role in calcium homeostasis, PTHrP is not pivotal in the development of milk fever.